The present invention generally relates to communications systems and, more particularly, to wireless systems, e.g., terrestrial broadcast, cellular, Wireless-Fidelity (Wi-Fi), satellite, etc.
Recently, Cognitive Radio (CR) (e.g., see, J. Mitola III, “Cognitive Radio: An Integrated Agent Architecture for Software Defined Radio,” Ph.D. Thesis, Royal Institute of Technology, Sweden, May 2000) has been proposed to implement negotiated, or opportunistic, spectrum sharing to provide a viable solution to the problem of sparsity of the wireless spectrum. To operate CR properly, it is important to perform spectrum sensing, i.e., the ability to detect licensed signals in their assigned spectrum bands. As a result, spectrum sensing becomes one of the core technologies of CR. The most challenging part of performing spectrum sensing is sensing signals in very low signal-to-noise ratio (SNR) conditions.
In this regard, a Wireless Regional Area Network (WRAN) system is being studied in the IEEE 802.22 standard group. The WRAN system is intended to make use of unused television (TV) broadcast channels in the TV spectrum, on a non-interfering basis, to address, as a primary objective, rural and remote areas and low population density underserved markets with performance levels similar to those of broadband access technologies serving urban and suburban areas. In addition, the WRAN system may also be able to scale to serve denser population areas where spectrum is available. Since one goal of the WRAN system is not to interfere with TV broadcasts, a critical procedure is to robustly and accurately sense the licensed TV signals that exist in the area served by the WRAN (the WRAN area).
In the United States, the TV spectrum currently comprises ATSC (Advanced Television Systems Committee) broadcast signals that co-exist with NTSC (National Television Systems Committee) broadcast signals. The ATSC broadcast signals are also referred to as digital TV (DTV) signals. Currently, NTSC transmission will cease in 2009 and, at that time, the TV spectrum will comprise only ATSC broadcast signals. However, in some areas of the world, instead of ATSC-based transmission, DVB (Digital Video Broadcasting)-based transmission may be used. For example, DTV signals may be transmitted using DVB-T (Terrestrial) (e.g., see ETSI EN 300 744 V1.4.1 (2001-01), Digital Video Broadcasting (DVB); Framing structure, channel coding and modulation for digital terrestrial television). DVB-T uses a form of a multi-carrier transmission, i.e., DVB-T is OFDM (orthogonal frequency division multiplexing)-based.
Since, as noted above, one goal of the WRAN system is to not interfere with those TV signals that exist in a particular WRAN area, it is important in a WRAN system to be able to detect DVB-T broadcasts (licensed signals) in a very low signal to noise ratio (SNR) environment. For an OFDM signal comprising N sub-carriers with sub-carrier spacing as Fs/N (Hz), its symbols in the time domain can be represented by samples with sample rate Fs (Hz). As known in OFDM transmission, each OFDM symbol includes a cyclic prefix (CP) to mitigate the affects of inter-symbol-interference (ISI). An example of an OFDM symbol 10 is shown in FIG. 1. OFDM symbol 10 comprises two portions: a symbol 12 and CP 11. The symbol 12 comprises N samples. The CP 11 consists simply of copying the last L samples (portion 13 of FIG. 1) from each symbol and appending them in the same order to the front of the symbol. As can be observed from FIG. 1, the symbol length of an OFDM symbol, M, is: M=N+L; where N is the number of subcarriers and L is the length of the cyclic prefix (CP). In this regard, the subcarriers used in an OFDM system and the length of the CP can be dynamically varied according to particular channel conditions. In particular, as shown in Table One of FIG. 2, a DVB-T signal can be transmitted in accordance with any one of eight transmission modes, each transmission mode having a different number (N) of subcarriers and CP length ratio (α), i.e., the ratio of the CP length over the symbol length N. For example, in transmission mode 1, the number of subcarriers, N, is equal to 2048 (2K mode) and the length ratio of the CP is 1/4, i.e., the CP consists of L=1/4 (2048)=512 samples. Similarly, in transmission mode 6, the number of subcarriers, N, is equal to 8192 (8K mode) and the length ratio of the CP is 1/8, i.e., the CP consists of L=1/8(8192)=1024 samples.